Since carbon fibers have excellent characteristic properties such as high strength, a high elastic modulus, high conductivity and light weight, they are used as fillers for high-performance composite materials. They are used as reinforcing fillers for the improvement of mechanical strength as in the prior art and further expected to be used as electromagnetic shielding materials, conductive resin fillers for antistatic materials and fillers for the electrostatic coating of a resin to use electric conductivity which the carbon fibers have. Making use of their features such as chemical stability, thermal stability and a micro-structure as a carbon material, they are expected to be used as field electron emitting materials in a flat display or the like.
As means of manufacture a carbon fiber for high-performance composite materials, (1) a process of manufacturing a carbon fiber by a vapor phase method and (2) a process of manufacturing a carbon fiber by melt spinning a resin composition have been reported.
As the process of manufacturing a carbon fiber by a vapor phase method, there are disclosed one in which an organic compound such as benzene is used as a raw material, an organic transition metal compound such as ferrocene is introduced as a catalyst into a high-temperature reaction furnace together with a carrier gas, and a carbon fiber is produced on a board (refer to JP-A 60-27700, particularly pages 2 to 3), one in which a carbon fiber is produced by a vapor phase method in a suspended state (refer to JP-A 60-54998, particularly pages 1 to 2) and one in which a carbon fiber is grown on the wall of a reaction furnace (refer to U.S. Pat. No. 2,778,434, particularly pages 1 to 2).
Although the carbon fibers obtained by these processes have high strength and a high elastic modulus, they are mostly branched and very inferior in performance as a reinforcing filler. Further, they have a high metal content because a metal catalyst is used. Therefore, when they are mixed with a resin, they deteriorate the resin by a catalytic function.
Meanwhile, as the process of manufacturing a carbon fiber by melt spinning a resin composition, one in which an extrafine carbon fiber is manufactured from a composite fiber of a phenolic resin and polyethylene (refer to JP-A 2001-73226, particularly pages 3 to 4) is disclosed. Although a carbon fiber having a small number of branched structures is obtained by this process, the phenolic resin is completely amorphous and therefore is hardly aligned and difficult to be graphitized, thereby making it impossible to expect the development of the strength and elastic modulus of the obtained extrafine carbon fiber.